Chakothi is 10kms from the Line of Control that divides Pakistani and Indian-administered Kashmir. Militant groups opposed to India are based in the region.

Pakistan says 57,000 people were killed in the 8 October earthquake and three million made homeless.

'Misheard'

The US helicopters are part of a huge international aid effort trying to bring relief to victims before the harsh Himalayan winter sets in.

The US military said: "A United States Army CH-47 Chinook helicopter flying in the vicinity of Chakothi delivering relief aid to earthquake victims is believed to have been fired upon by a rocket-propelled grenade today."

It said the helicopter returned safely to base at Chaklala at 1430 and an investigation was under way.

"Our investigation revealed that it was blasting on the roadside by engineers, under way exactly at that time when the helicopters was flying over the area," Gen Sultan said.

He said the US was "satisfied with the explanation".

However, US Commander Nick Balice at the Disaster Assistance Centre public affairs office in Islamabad, said: "Our air crew is familiar with RPG fire,"

Before the incident, the US military had promised to keep flying its helicopters in northern Pakistan and Pakistan-administered Kashmir through the winter and urged other nations to continue their efforts too.

Rear Adm Mike LeFever, commander at the Disaster Assistance Centre, said: "We are not going to diminish our helicopter support. This is long-term support. We are going to be standing by our friends, and we expect the other international communities to be able to do that."

His stance was reiterated by Commander Balice after the Chinook incident. He said it would not affect flights.

Pakistan's official death toll from the quake is now 57,597, with 78,800 injured.

More than two-thirds of the casualties were in Pakistan-administered Kashmir while 18,000 died in North-West Frontier Province.